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Ambient or incident-light meter

How to take an ambient-light meter reading


Minolta's Auto Meter IV F is an example of a top-rated incident light meter. The meter's white dome is held in the light striking the subject and pointed towards the camera when taking a reading.
Minolta's Auto Meter IV F is an example of a top-rated incident light meter. The meter's white dome is held in the light striking the subject and pointed towards the camera when taking a reading.

Almost all new digital, 35mm and medium-format cameras sold today come with a built-in light meter. Many experienced photographers, however, continue to use a handheld accessory exposure meter, known as an incident light meter or an ambient light meter that measures light in a different manner.

A camera's built-in meter reads the light reflected by the subject; an incident-light meter reads the light falling onto the subject. The camera must be pointed at the subject in order for its built-in meter to make a reflected reading, whereas a handheld incident-light meter is usually pointed from the subject towards the camera to read the light striking the subject.

Properly-used, incident-light meters give the most-accurate information about the amount of light.

This page deals only with the incident-light meter and its application. It begins with how to take an incident or ambient-light meter reading.

FIRST SET THE FILM SPEED OR YOUR DIGITAL CAMERA'S ISO SENSITIVITY SETTING

With a fresh battery installed in your incident (or ambient) light meter and with the power button turned on, the first step in taking a light reading is to set the film speed or the ISO sensitivity setting for your digital camera.

Various light meters have different means of setting film speed or ISO settings, and we recommend you refer to the operating manual supplied with your light meter. A common method is to depress the ISO button (if so equipped) on the light meter and use the meter’s up/down control switch to select the ISO value of your film. If the film or sensitivity setting in your camera is ISO 100, select 100 on your meter.


It is not unknown for photographers to forget to change the film speed or ISO setting on a hand-held light meter when you switch to a faster or slower film in your camera or a different sensitivity setting in your digital camera, particularly if you use one speed of film or one sensitivity setting a lot. Sometimes you may accidentally hit the wrong button, inadvertantly changing the film speed in your meter, and not be aware of it. A good practice is to check your meter’s film speed indicator frequently - at least with every change of a roll of film or a change in sensitivity setting.

SECOND, SET THE SHUTTER SPEED, IF NECESSARY

An incident-light meter generally provides a reading that is translated into exposure values (EV) and shows several suggested combinations of shutter speeds and aperture settings, each of which will give correct exposure.

Some incident-light meters, however, will also display the correct f/number (aperture size) for a specific pre-set shutter speed after taking a reading. For these meters, the shutter speed you intend to use should first be entered in the meter. This is usually accomplished by pressing the up/down switch until the correct shutter speed appears on the meter’s display panel. Top-quality light meters will have a shutter speed range from 1/8000 to 30 seconds in -stop increments.

For exposure meters that display only EV numbers, the shutter speed does not need to be pre-set. Still other meters can be set to display either EV numbers or the recommended f/stop for a pre-determined shutter speed.

TAKING AN INCIDENT-LIGHT METER READING

To take an incident-light reading, be sure your meter is set to ambient mode, if necessary. Position the meter near your subject, aim the receptor - generally a silicon photo cell located under a translucent white diffuser that is commonly dome-shaped (for three-dimensional subjects), but may also be flat (for photography of flat surfaces, like a painting's) - directly at the camera, and press the measuring button. The dome (also known as a diffusing hemisphere or disc) over the photocell "averages" the light falling on it.

An ambient light meter will provide the same reading regardless of the background's reflectiveness.
An ambient light meter will provide the same reading regardless of the background's reflectiveness.

Whether the background is black or white or a highly-reflective mirror surface, an ambient lightmeter reads only the light striking it, not reflected from it.
Whether the background is black or white or a highly-reflective mirror surface, an ambient lightmeter reads only the light striking it, not reflected from it.

Some photographers recommend placing the dome at an angle between the direction of the light source and the camera when taking an incident-light reading to obtain satisfactory results.

When a flat diffuser is fitted to the light meter, the meter is first pointed at the camera from the subject, then at the brightest source of illumination from the subject, and the two readings are averaged. This is called the Duplex Method.

Many meters will continue to make readings as long as the measuring button is kept depressed, and the last reading will remain in the display when the button is released.

If you are unable to approach a sunlit subject because it is some distance away, across a river or road, for example, simply take a reading of the same sunlight that is striking the subject from where you currently are. This is known as a "substitute reading."

In a few situations, you may need to use slightly different exposure settings than those suggested by an incident-light meter. A highly-reflective surface, such as a ski hill in winter or an outdoors rink, will require a decrease in exposure settings so that its surface texture is captured on film. Conversely, a very dark subject, like a black bear in the shade, will require a slightly-increased exposure to give its fur some texture.

If the reading is over or under the meter’s display range or measurement range, this will usually be indicated on the meter’s display.


WHAT IF THE SUBJECT IS NOT EVENLY LIT?

If you measure only the light falling on the brightly-lit side of a subject in high-contrast lighting, and take a picture based on that exposure reading alone, there may be insufficient detail in the subject’s shadow side. Whereas a single meter reading from the shadow side may bring out the detail there, but will overexpose the subject’s bright side, washing out the highlights.

The answer is to take an average of both readings. If you obtain an exposure reading that calls for an aperture of ƒ/8 on the subject’s shadow side and an aperture of ƒ/16 on the subject’s bright side, the average exposure is ƒ/11 – right between the two. A picture taken at ƒ/11 would likely provide sufficient detail in both sides of your subject, dependent upon your film’s exposure range.

(A film’s exposure range determines how bright and dark the areas of a scene can be for the film to still record detail, and it is generally found in the information material packaged with the film. In general, color negative film has an exposure range of approximately seven stops, and color slide film has only about five stops.)

Some meters with built-in memories, like the Minolta IV F, will automatically calculate averaged exposure readings for you. You simply measure and store readings of the highlight and shadow areas, and press the meter’s “average” button.

Meter readings for subjects that are backlit should be based on the light that is less bright in front. A reading of the bright back-lighting will cause your subject to be under-exposed in front, because the front is in shadow.

When the brightest light comes from behind your subject, it is called backlighting, and requires you to take a meter reading of the front light falling on your subject in the darker shadow area.
When the brightest light comes from behind your subject, it is called backlighting, and requires you to take a meter reading of the front light falling on your subject in the darker shadow area.

An incident light meter does not
An incident light meter does not "know" it when you have placed a light-reducing filter on your lens, and you must compensate for it when you take a meter reading.

FILTER FACTOR CORRECTION

A filter placed on a lens can reduce the amount of light reaching the film or digital camera's sensor. Most filters absorb some light – some more than others. Since a hand-held incident light meter is not built into the camera, it will measure the light as if there is no filter attached. To compensate for this, you will have to increase the exposure that is indicated by the light meter. (See our section on Filter factor for information on how to do this.)

Some light meters permit you to add the filter factor directly to the film speed setting so that the meter will automatically compensate for the light absorbed by the filter (or filters, if more than one has been attached to your lens).


Further information...

Care of your incident-light meter